1. Field
The present invention relates to structural aircraft parts. More particularly, the present invention relates to a load-bearing structure, such as a pylon or a strut, for supporting an aircraft engine on an airframe structure of an aircraft.
2. Related Art
A pylon (or strut) that attaches a high-bypass engine to the wing of a commercial airliner is a complex structure that is very highly loaded and performs a multitude of functions such as supporting the engine weight, the fairings and systems, providing a fire and vapor barrier between the engine and the wing, transmitting the engine thrust into the structure of the airplane, and supporting the engine nacelle and thrust reverser in the optimum aerodynamic location.
The pylon must be lightweight and as small as possible to avoid exacting a large aerodynamic penalty. A typical pylon is usually composed of twenty-five or more major parts. These parts often require shims in order to be assembled and are held together by hundreds or even thousands of fasteners. Additionally, extensive corrosion protection and sealing of joints and fasteners are required. The cost of such units is a considerate portion of the total airplane price, despite the structure being a relatively small fraction of the airplane's total mass. The aircraft industry has struggled to reduce the cost of this assembly while maintaining the highly redundant structure that assures this critical component will not fail despite its being located in an environment of high loading, high sonic fatigue, high temperatures, and corrosive gasses.
The primary load carrying elements of current pylons, used to support large fan engines under the wings of commercial airplanes, have been composed of multiple metal pieces held together with mechanical fasteners. Typically this assembly consists of a semi-monocoque structure where the loads are carried in chords and webs supported by frames and bulkheads. Many of these components are made from titanium or corrosion-resistant steel in order to withstand high temperatures and a severe fatigue spectrum. The use of these materials, instead of aluminum, raises the cost, both in the fabrication of the parts and in the assembly stage, where hundreds of holes must be drilled through these very tough materials. Further, the use of mechanical fasteners requires considerable overlapping of the joining surfaces which adds weight and cost.
A more efficient process for creating such highly loaded structure would be to produce it in a manner that would significantly reduce the part count, eliminate inefficient load paths and part to part overlaps, and drastically reduce fastener usage. In the past, the technology did not exist to produce a structure of this size, complexity and strength by any means except as described in the paragraphs above.
However, increasingly, the material of choice in aerospace application is some form of composite, usually composed of graphite in an epoxy matrix. This lighter weight material is enabling designers to reduce the weight of aircraft while, at the same time, keeping costs down due to lower assembly times and less dependence on costly metals, such as titanium, where the basic raw material costs are rising rapidly with demand outpacing supply. Composites have long been used in the propulsion installation that supports the large fan engines used on current airlines, but that usage has been largely limited to the nacelle, where the major loads have been “hoop tension” loads that the composites are ideally suited to accommodate, or in the fairings covering the pylon, where the loads are relatively small. There has been no application of these materials into the actual torque box structure of the pylon where the reversible loads and highly concentrated loads at the input points often favor the use of metals.
The pylon is a prime example of “primary structure” which is a term used to refer to those portions of the aircraft that cannot be allowed to fail without putting the entire airplane in danger of being lost. It is important then that this structure is inherently fail-safe. This means that the structure must be designed to continue to function as intended despite any reasonable damage and it must continue to function through the years despite a very harsh fatigue spectrum. Composite materials, due to their inherent nature, are ideal for such applications, but composites have not been used in this application before because they do not react well to high bearing stresses, and numerous points on the pylon are subject to just that type of loading.
Accordingly, there is a need for an improved load bearing structure for supporting the engine of an aircraft that does not suffer from the problems and limitations of the prior art.